A U.N.-hosted pledging event, organised along with the governments of Ethiopia and the United Kingdom and held at the United Nations in Geneva, is designed to increase life-saving assistance to 15.5 million people and food aid to 10.4 million.

"The emergency has been building up through cycles of droughts and floods, and conflict," the U.N. humanitarian office (OCHA) said.

"El Nino (weather pattern) has exacerbated a drought in the northern highlands and millions of people are coping with less water, drier pastures and smaller harvests."

OCHA said that conflict and climate shocks had caused more than 21 million people in Ethiopia to require humanitarian aid this year, and that 10.8 million people were projected to be critically food insecure during the lean season between July and September.

OCHA said that 21 countries had made pledges amounting to $628.9 million. The biggest donor was the United States with $253,000 followed by the United Kingdom with 100 million pounds($124.58 million) and the European Union with $46.6 million.

Andrew Mitchell, Britain's deputy foreign secretary and minister for development and Africa, said earlier he hoped the pledge would encourage other countries, including in the Gulf region, to take part. However, none were on the donor list.

A two-year war that erupted in November 2020 between the federal government and forces led by the Tigray People's Liberation Front, which dominates the northern region, has killed tens of thousands of people, created famine-like conditions for hundreds of thousands, and displaced millions.

($1 = 0.8027 pound)

(Reporting by Gabrielle Tétrault-Farber and Emma Farge; editing by Jonathan Oatis, Alexandra Hudson)